The field of the invention is cortical implants. More particularly, the invention relates to high-resolution, untethered flexible cortical implants.
Optogenetics, which uses light stimulation to control the excitation, inhibition, or signaling pathways of optically excitable cells in genetically modified neural tissue, provides a powerful tool to diagnose and treat, as well as understand, numerous neurological and psychiatric diseases and disorders such as epilepsy, stroke, seizures, paralysis, depression, schizophrenia, Parkinson's disease, and Alzheimer's disease. Using a viral vector, neurons are genetically modified to express light-gated ion channels in the cellular membrane sensitive to incident light. Cells expressing Channelrhodopsin-2 (ChR2) are activated or excited by blue light, while yellow light directed on cells that express Halorhodopsin are quieted or silenced.
Optogenetics may be divided into two different methodologies for neurological optical stimulation. One approach requires inserting the exposed tip of a fiber optic cable(s) into brain tissue. An external light source, either a laser or light-emitting diode (LED), is then mechanically connected to the unexposed end of the fiber-optic cable, and the neural tissue, both surrounding and underneath the exposed tip, is illuminated. Adding a connector to the short length of fiber-optic cable protruding from the cranial insertion point provides reasonable freedom of movement when the light source is not attached and activated. However, penetrating fiber-optic-based neural stimulation methods still require a permanent opening in the cranium, which poses serious infection risk. Additionally, the single cylindrical fiber light source from the exposed tip projects non-specific omni-directional illumination over a large volume of neural tissue. Thus, the ability of fiber-optic-based methods to target specific neural regions is limited.
One solution to the fiber optic related limitations described above includes inserting a small array of discrete LEDs bonded to the surface of a mechanically compliant thin biocompatible substrate, which also provides the necessary electrical interconnections. Assuming a suitable miniature electrical power source for the LEDs can be mounted inside the skull, this approach is designed to eliminate the need for the permanent opening, as well as provide more directional and localized illumination. The array of sub-cranial LEDs can then be placed in direct contact with either the cortical surface or the deep brain using a penetrating probe with a smaller linear LED array. The surface-mounted, flexible, discrete LED array is designed to conform to the uneven or folded surface structures of the cerebral cortex for direct optical stimulation, or can be positioned in the narrow and deep crevice which separates the forebrain into its left and right cerebral hemispheres.
There are assumed to be significant clinical advantages to selectively (optically) exciting small isolated regions of neural tissue, as opposed to activating an entire emissive array. However, discrete LED-based methods are limited in resolution given the requirement to individually bond each discrete LED to the supporting flexible substrate, which quickly becomes unmanageable as the resolution is increased. More importantly, as the LED array size (i.e., x-y matrix resolution) increases beyond a few LEDs, the ability to individually activate each discrete LED is no longer possible unless a major portion of the display area is converted to interconnect wiring/traces.
For example, to individually connect to or address every emissive pixel in an array with x rows and y columns requires connections amounting to the product of x and y. If the reported deep-brain-penetrating implant is a parallel linear array of four LEDs, for example, with a common cathode and anode connection, to activate each pixel individually, the anode connection must be split into four separate leads. However, the typical minimum metal trace pitch for high density flex printed circuit boards (PCBs) is 50 μms. Adding the required additional metal traces nearly doubles the original width of the probe for just four anode connections. Thus, the flex PCB interconnect requirements to individually address even a small 32×32 discrete LED array, for example, would require 1024 separate metal interconnects, which is unmanageable from a design and manufacturing standpoint. In addition, such a device would require greater electrical power consumption from a power source placed inside the skull. It is also possible that a large number of LED's in a long-term implant would result in undesirable tissue heating.
Thus, there is a need for a system and method that selectively activates only a small subset of the overall emissive optical array. In addition, there is also a need for a system that requires less power than conventional optogenetics devices, as power consumption is directly proportional to the number of active emitting LEDs. A system that requires a decreased power consumption is also needed, such that it would be possible for a wireless inductive power source to be used, removing the expected challenges associated with supplying power from a miniature source placed inside the skull.